1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus using a photoreceptor including a specific charge transport material, and to a process cartridge including the photoreceptor.
2. Discussion of the Background
Recently, development of information processing systems utilizing electrophotography is remarkable. In particular, optical printers in which information converted to digital signals is recorded using light have been dramatically improved in print qualities and reliability. This digital recording technique is applied not only to printers but also to copiers, and so-called digital copiers have been developed and used. Copiers utilizing both the conventional analogue recording technique and this digital recording technique have various information processing functions, and therefore it is expected that demand for such copiers will be escalating. In addition, with popularization and improvement of personal computers, the performance of digital color printers which can produce documents.
In place of conventional inorganic materials such as Se, CdS and ZnO as photoconductive materials for electrophotographic photoreceptors, organic photoconductive materials having better sensitivity, thermostability and nontoxicity than the inorganic materials are now prevailing. Most of the electrophotographic photoreceptors using the organic photoconductive materials have functionally-separated photosensitive layers formed of charge generation layers including charge generation materials and charge transport layers including charge transport materials.
On the other hand, a single-layered photoreceptor including a charge generation material and a charge transport material in a single photosensitive layer now attracts attention because it can be prepared by a simple process, optical properties improve due to fewer layer interfaces and it can be used for either a positive or a negative charging process.
Typically, an electrophotographic image forming apparatus irradiates writing light modulated by image data onto a uniformly charged photoreceptor to from an electrostatic latent image thereon; and provides a toner to the photoreceptor from the image developer to develop the electrostatic latent image to form a toner image thereon.
As charging methods in the electrophotographic image forming apparatus, a corona charging method using a metallic wire and contact charging method contacting a charging roller to the photoreceptor, etc. are known.
The contact charging method produces less oxidizing gases such as ozone and NOx than the corona charging method because a lower voltage can be applied to the charging member of the contact charging method than that applied to that of the corona charging method. However, the contact charging method having high discharge energy gives a large stress to a photoreceptor and the photoreceptor is abraded faster, has poor charge followability when the process linear speed becomes faster and the photoreceptor tends to have discharge breakdown because a high voltage is directly applied thereto.
The corona charging method having good charge following capability and less electrostatic stress to a photoreceptor, and therefore is advantageous for higher speed and durability of the apparatus, but produces more oxidizing gas such as ozone and NOx (about 100 times as much as that produced by the contact charging method).
When a photoreceptor is exposed to an oxidizing gas such as ozone and NOx, a charge generation material and a charge transport material in a photosensitive layer thereof are oxidized, which causes decrease of chargeability, increase of residual potential and deterioration of sensitivity thereof, resulting in production of images having low image density and foggy images.
In order to solve these problems, Japanese Laid-Open Patent Publication No. 57-122444 discloses a method of including an antioxidant in the photosensitive layer. However, when an antioxidant is included therein, there are adverse effects of increase of residual potential and deterioration of sensitivity thereof from the beginning or when repeatedly used, resulting in insufficient durability thereof.
Because of these reasons, a need exists for a photoreceptor without deterioration such as decrease of chargeability due to an oxidizing gas which is massively produced when the corona charging method is used, which is capable of preparing a high-speed and high-durability electrophotographic image forming apparatus.